Suppress scanner errors for non-existent URIs.

pkg/analyzer/lib/src/task/dart.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.task.dart;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/ast.dart';
@@ skipping 3794 matching lines @@
     if (context.getModificationStamp(target.source) < 0) {
       String message = 'Content could not be read';
       if (context is InternalAnalysisContext) {
         CacheEntry entry =
             (context as InternalAnalysisContext).getCacheEntry(target);
         CaughtException exception = entry.exception;
         if (exception != null) {
           message = exception.toString();
         }
       }
-      errorListener.onError(new AnalysisError(
-          source, 0, 0, ScannerErrorCode.UNABLE_GET_CONTENT, [message]));
+      if (source.exists()) {
+        errorListener.onError(new AnalysisError(
+            source, 0, 0, ScannerErrorCode.UNABLE_GET_CONTENT, [message]));
+      }
     }
     if (target is DartScript) {
       DartScript script = target;
       List<ScriptFragment> fragments = script.fragments;
       if (fragments.length < 1) {
         throw new AnalysisException('Cannot scan scripts with no fragments');
       } else if (fragments.length > 1) {
         throw new AnalysisException(
             'Cannot scan scripts with multiple fragments');
       }
@@ skipping 299 matching lines @@
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }